AUTHOR=Suenaga Toshikazu , Riya Shohei , Hosomi Masaaki , Terada Akihiko 

TITLE=Biokinetic Characterization and Activities of N2O-Reducing Bacteria in Response to Various Oxygen Levels

JOURNAL=Frontiers in Microbiology

VOLUME=9

YEAR=2018

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2018.00697

DOI=10.3389/fmicb.2018.00697

ISSN=1664-302X

ABSTRACT=<p>Nitrous oxide (N<sub>2</sub>O)-reducing bacteria, which reduce N<sub>2</sub>O to nitrogen in the absence of oxygen, are phylogenetically spread throughout various taxa and have a potential role as N<sub>2</sub>O sinks in the environment. However, research on their physiological traits has been limited. In particular, their activities under microaerophilic and aerobic conditions, which severely inhibit N<sub>2</sub>O reduction, remain poorly understood. We used an O<sub>2</sub> and N<sub>2</sub>O micro-respirometric system to compare the N<sub>2</sub>O reduction kinetics of four strains, i.e., two strains of an <italic>Azospira</italic> sp., harboring clade II type <italic>nosZ</italic>, and <italic>Pseudomonas stutzeri</italic> and <italic>Paracoccus denitrificans</italic>, harboring clade I type <italic>nosZ</italic>, in the presence and absence of oxygen. In the absence of oxygen, the highest N<sub>2</sub>O-reducing activity, <italic>V</italic><sub>m,N2O</sub>, was 5.80 ± 1.78 × 10<sup>−3</sup> pmol/h/cell of <italic>Azospira</italic> sp. I13, and the highest and lowest half-saturation constants were 34.8 ± 10.2 μM for <italic>Pa. denitirificans</italic> and 0.866 ± 0.29 μM for <italic>Azospira</italic> sp. I09. Only <italic>Azospira</italic> sp. I09 showed N<sub>2</sub>O-reducing activity under microaerophilic conditions at oxygen concentrations below 110 μM, although the activity was low (10% of <italic>V</italic><sub>m,N2O</sub>). This trait is represented by the higher O<sub>2</sub> inhibition coefficient than those of the other strains. The activation rates of N<sub>2</sub>O reductase, which describe the resilience of the N<sub>2</sub>O reduction activity after O<sub>2</sub> exposure, differ for the two strains of <italic>Azospira</italic> sp. (0.319 ± 0.028 h<sup>−1</sup> for strain I09 and 0.397 ± 0.064 h<sup>−1</sup> for strain I13) and <italic>Ps. stutzeri</italic> (0.200 ± 0.013 h<sup>−1</sup>), suggesting that <italic>Azospira</italic> sp. has a potential for rapid recovery of N<sub>2</sub>O reduction and tolerance against O<sub>2</sub> inhibition. These physiological characteristics of <italic>Azospira</italic> sp. can be of promise for mitigation of N<sub>2</sub>O emission in industrial applications.</p>